As a sustainable way to utilize water resources, solar desalination has great potential in solving the problem of insufficient fresh water resources and alleviating water shortages. However, traditional seawater desalination technology often has problems such as high energy consumption, complex equipment, and high cost.
In recent years, new desalination coating technologies based on nanomaterials have gradually attracted the attention of researchers. Graphene oxide reduced graphene, as a new two-dimensional material, has shown significant potential in solar desalination.
Compared with traditional desalination coating materials, rGO has the advantages of excellent conductivity, high specific surface area and good chemical stability, and is expected to improve desalination efficiency and reduce energy consumption.
However, rGO itself has problems of aggregation and deposition, which reduces its activity and stability. Therefore, fixing rGO on foam materials to form a melamine foam/rGO composite coating has become an effective way to improve desalination efficiency and stability.
By surface modification and strengthening the carrier properties of the foam material, we uniformly fixed rGO on the foam surface to increase its contact area with salt ions in the water. At the same time, the structure and thickness of the rGO coating are controlled to optimize desalination efficiency and stability.
To gain a deeper understanding of the desalination mechanism, we also prepared a reduced graphene oxide (rGO) foam. Graphene trioxide-coated melamine foam has an inherent porous microstructure and hydrophilicity, which allows it to have good wettability and can attract water to the evaporation area, allowing water to be replenished quickly while avoiding salt precipitation.
The interfacial evaporation system of graphene oxide-coated melamine foam can achieve a steady-state evaporation efficiency of 89.6% under 1kW m−2 solar fuel. This interfacial evaporation system has high solar-to-heat conversion efficiency and good long-term stability, showing potential for commercial seawater desalination.
At present, the global energy crisis and water shortage have become major threats to the sustainable development of human society. To meet the growing energy demand, the development of various high-performance energy conversion and water purification technologies has received increasing attention.
Solar powered interfacial evaporation technology
As a promising renewable energy source, solar energy is due to its ultra-large capacity, universality, cleanliness, and multi-functional conversion capabilities. It has shown the potential to replace fossil fuels retained by fossil fuels.
In addition to solar photovoltaic technology directly generating electrical energy, solar-thermal energy conversion technology also absorbs solar energy and converts it into thermal energy, which is considered a simple and effective way to utilize solar energy.
The thermal energy converted from solar energy can drive important industrial processes such as solar thermal power stations, solar desalination, sterilization, solar water heating systems and other heat-related applications. Solar desalination offers an effective solution, especially for areas lacking infrastructure, because it can extract fresh water from seawater without additional energy input.
In recent years, solar-driven interfacial evaporation technology locates the solar-heat conversion process at the liquid-vapor interface and has become a new solar energy utilization technology. Compared with traditional integral heating, interfacial evaporation systems exhibit higher evaporation efficiency and faster thermal response and are a potential alternative for seawater desalination.
So far, many studies have been carried out on improving the thermal performance of interfacial evaporation systems. A typical solar-driven interfacial evaporation system consists of three important parts:
A solar absorber with high solar absorption and efficiently converts solar radiation into heat energy; an insulator that transfers the converted heat to the bulk water; and a water supply path that continuously draws water from the bulk water to the surface of the solar absorber.
In previous studies, a large number of researchers have devoted themselves to improving the overall solar-to-steam conversion efficiency by synthesizing broadband and solar energy absorbing materials, optimizing the design of new water supply paths, and improving thermal insulation structures.
So far, solar-driven interfacial evaporation systems have been used in many industrial processes, induced solar distillation, solar sterilization, solar desalination and solar power generation. To achieve high evaporation performance of the system, it is important to explore the broadband absorption across the solar spectrum of solar absorbing materials.
The solar energy absorbing materials developed and explored in solar-driven interfacial evaporation systems mainly include carbonaceous materials, plasmonic particles and spectrally selective absorbing materials. Among them, natural black and carbonaceous materials have strong solar-heat conversion capabilities and are suitable for broadband and high-intensity solar absorbers.
Due to their high solar absorption rate, low cost, and good stability, many carbon-based solar absorbers have been studied, including carbon nanotubes, graphite, and reduced graphene oxide. After years of development, the system’s solar-to-steam conversion efficiency under daylight has reached more than 90%.
Although there have been great improvements in developing solar energy absorbing materials and designing interfacial evaporation structures, most previously reported solar desalination devices were due to salt accumulation on the top surface of the evaporator during the desalination process. The accumulated salt blocks solar absorption and blocks the porous evaporator, resulting in low solar-to-heat conversion efficiency and poor evaporation performance.
Traditional physical methods such as ultrasonic waves and water immersion are generally used to remove accumulated salts, but these methods can easily cause damage to the interface evaporation structure.
To reduce the accumulation of salt in interfacial evaporation systems, many studies have focused on developing hydrophobic evaporation. However, hydrophobic evaporators limit the water supply to the top surface, thereby reducing the evaporative performance of the system. Another way to solve this problem is to use a hydrophilic evaporator, which moves excess salt from the top surface to the bulk water.
To date, many studies have focused on designing hydrophilic evaporators, such as hydrophilic cellulose fabric membranes, paper-based hydrophilic membranes, and bio-inspired evaporators. But developing a long-term stable and highly efficient solar-driven interfacial evaporation system remains a significant challenge.
In this study, we demonstrate a salt-resistant, highly efficient solar-absorbing and fast thermally responsive reduced graphene oxide (rGO) foam for solar-driven interfacial desalination under low-fluorescence solar illumination.
In the new porous structure, hydrophilic melamine foam (MF) was chosen as the base, which not only expands the surface area for solar absorption but also enhances multiple scattering.
The surface hydrophilicity and porosity of the reduced graphene oxide-coated melamine foam provide sufficient wetting and fusion of the solar heat conversion zone, as well as rapid replenishment of water, while avoiding the accumulation of salts.
Taking advantage of the high performance of reduced graphene oxide-coated melamine foam, the solar-powered interfacial system is capable of a solar-to-steam conversion rate of 89.6%.
Furthermore, by integrating reduced graphene oxide-coated melamine foam into commercial solar energy, we designed and fabricated a solar-driven interfacial seawater desalination device with a solar-to-water conversion efficiency of 56.4% under daylight. Due to solar desalination, the clean water collected is of drinkable quality requirement.
With superior evaporation performance and scalability of solar-to-thermal conversion materials, this system for solar-driven interfacial seawater desalination presents broad application prospects in practical applications of seawater desalination.
Scalability of solar desalination heat transfer materials
An interfacial evaporation system based on reduced graphene oxide-coated melamine foam. Ter graphene oxide-coated melamine foam is in the middle and is wrapped by a layer of polyethylene (PE) foam on the outside to construct a self-bubble interfacial evaporation system.
Melamine foam coated with tellurium-reduced graphene oxide is used to quickly absorb incident solar radiation, convert solar energy into heat, and generate a driving force for interfacial evaporation. On the other hand, hydrophilic reduced graphene oxide-coated melamine foam can continuously draw brine into the evaporation zone.
The porous structure of the graphene trioxide-coated melamine foam allows the generated vapor to escape into the environment, promoting the capture of sunlight and thus increasing the efficiency of solar-to-heat conversion. The reduced graphene oxide-coated melamine foam is porous and hydrophilic, allowing it to draw water into the evaporation structure described above while simultaneously breaking down concentrated salts back into the bulk seawater.
During the evaporation process, the salt concentration at the foam is higher than the concentration of sea water at the bottom. Changes in concentration cause the salt to break down into the bulk water. In addition, the surrounding polyethylene foam not only reduces the graphene oxide-coated melamine foam, but also naturally gathers on the water surface, inhibiting heat loss caused by the surrounding salt water.
Interface evaporation systems can operate continuously and efficiently on water surfaces without the need for complex construction designs. Furthermore, since evaporation occurs at the gas-liquid interface, the interfacial evaporation system can move autonomously when the interface recedes.
Tellurium-reduced graphene oxide-coated melamine foam is a key component of the interfacial evaporation system. Melamine foam was immersed in graphene oxide colloidal suspension to prepare reduced graphene oxide-coated melamine foam, and then graphene oxide was reduced into reduced graphamine oxide using a hydrothermal method.
Reduced graphene oxide-coated melamine foam with a diameter of 3.5 cm and a thickness of 1.6 cm is easy to fabricate and its color is deep black.
The roughness of the surface of the reduced graphene oxide-coated melamine foam material observed using a three-dimensional optical microscope showed a height variation of more than 300 μm. The rough surface structure of solar thermal conversion composites is mainly attributed to the porous structure of melamine foam, and the modified reduced graphene oxide also increases the surface roughness.
Surface roughness amplifies the solar absorption surface area while increasing multiple scattering of incident sunlight, thus improving the solar-to-heat conversion efficiency of the system. Microstructure of the prepared melamine foam at different magnifications. The graphene oxide-coated melamine foam surface exhibits a wrinkled and interconnected network structure.
The pore size of rGO-coated melamine foam is between 20 and 40 μm. The open pore structure and wrinkled surface facilitate the deposition of graphene oxide. The reduced graphene oxide-coated melamine foam has good water wettability due to the hydrophilicity of the porous melamine foam.
Once a water droplet touches the surface of the reduced graphene oxide-coated melamine foam, it is completely absorbed within 0.5 seconds. The hydrophilic surface of reduced graphene oxide composites can quickly guide drainage within the porous structure. The porosity of reduced graphene oxide-coated melamine foam was determined by comparing the mass changes before and after water absorption.
The porosity of graphene oxide-coated melamine foam can reach 85.55%. Solar energy absorption by reduced graphene oxide-coated melamine foam directly affects the solar-to-heat conversion efficiency of the system. To evaluate the salt resistance, reduced graphene oxide-coated melamine foam was placed in sodium chloride solution (3.5 wt%).
Then 3 g of solid sodium chloride was placed on the upper surface and the Tergo-coated melamine foam was placed at a solar density of 1 kW m−2. The amount of solid salt gradually dissolves after 120 minutes.
When sunlight shines on the top surface of the rGO-coated melamine foam, the temperature of the rGO-coated melamine foam rises rapidly within minutes and remains at 35°C throughout the evaporation process. Since the solubility of salt depends on temperature, the higher the temperature, the faster the salt dissolves and therefore the salt iron can be redissolved into the surface water.
In reduced graphene oxide-coated melamine foam, which has numerous pores with high water absorption capacity that can provide a continuous water supply to the top surface, salts can move from the top surface to the sublayer of water. Temperature changes lead to salinity changes between the upper evaporation surface and the body.
In order to obtain broadband and efficient solar energy absorption, melamine foam was modified with porous hydrophilic melamine foam, thereby increasing the solar absorption surface area and increasing multiple scattering.
The solar interfacial evaporation system based on rGO-coated melamine foam exhibits rapid thermal response at a solar power density of 1 kW m−2, achieving a steady-state solar thermal conversion efficiency of 89.6%.
Finally, we integrated a reduced graphene oxide-coated melamine foam evaporator into a solar still, which can produce 56.4% drinking water with salt accumulation at 1 kW m−2.
We envision that this solar-powered interfacial desalination device could be used on isolated islands or in the ocean to collect drinking water from abundant seawater or other saltwater resources.
Conclusion
Desalination rGO-coated melamine foam, as an innovative and efficient solar desalination technology, has great potential and application prospects. It can not only provide reliable fresh water resources for water-scarce areas, but also reduce dependence on traditional energy, reduce energy consumption and environmental pollution.
The results of this research will make an important contribution to solving the global freshwater resource shortage problem and promote the realization of sustainable development.
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